Synthetic modulator system



R. A. HILFERTY SYNTHETIC MODULATOR .SYSTEM June l2, 1945.

Filed Sept. 18, 1943 Sheets-Sheet 1 l Qu zal.. Qns

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AT1-ORNE duzz (am June 12, 1945 R. A. HILFERTY SYNTHETIC MQPULATORSYSTEM 2 Sheets-Sheet 2 Filed Sept. 18, 1945 ATTOR Patented ilune1?,.1-945 l UNITED STATES Richard A. Hilferuf, BaldwinyN'; y.; assieme-0Press Wireless, Inc., Chicago, Ill., avcorporation of DelawareApplication september'is, 1943,serial No; 502,870 y 6 claims.` (ci.25o-frz),

This invention relates to radio transmitter and receiver systems andmore especially to systems where intelligence signals are representedbyk tone or audio frequencies. 'i f A principal object'of the inventionis' to'provide aynovel system of transmitting facsimile signals and thelike without using' expensive audio frequency modulating equipmentat thetransmit-l ter. i One very wellknown method of transmitting facsimile orsimilar signals bycrne'ans of`a 'radio transmission link, is to convertthe' shade'values of the original subject matter into 'audio frequencytones which either directly or indirectlyjare used to amplitude-modulatear radio frequency carrier. Such a method requires expensiveampli-tude-modulating equipment at the transmitter for amplitudemodulation of the carrier. Not only are such modulators expensive asregards initial costland installation, .but they are seldom used' attheir full frequencyutilizing capacity. be-

cause the audio frequency modula-ting'band to be controlled shiftablfrequencywhichcan be transmitted is usually very narrow, e, .g., 1500-3000 C. P. S., ascompared with thefaudio modu-l lating equipmentwhich'isusually designed for operationbetween 50 and 10,000 C.`P. S.

Accordingly, another object is to provide' a system of transmittingdifferent audio frequency tone signals by means'o'f radio carrierswithout using conventional audio modulator equipmentat the transmitter,while 'allowing reception to take place with conventionalradio receiversof the am'- plitude-demodulation kind. A feature of the inventionrelates to a flexible service radio system employing a pair ofnonamplitude-modulated transmitters whose relative frequency differenceis varied in a desired audio frequency manner; these transmitters canalternatively be used in the conventional manner as independenttelegraph-keyed transmitters.

Another feature relates to the novel organization, arrangement andrelative interconnection of partswhich cooperate to produce a novel andexible radio service system.

Other features and advantages not' specifically enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claims. In the drawings,

Fig. 1 represents in schematic block diagram form, a radio transmitterand receiver system according to the invention.

Fig. 2 is a modification of Fig. 1.

Fig. 3 is another modification. y

Referring to Fig. 1, theradio frequency excitation portion of thetransmitter comprises two separatesradio frequency oscill'ators'l and2,' which may be of any Well-'kri'ovvn-type, and if desired,providedfwith suitable" amplifiers *and 'buffers as Well-known "in theradio trjansmitterl art. *The frequency 'of oscillator l ymay befiixedfor example at 15,639 kilocycles, While-oscillatoris `of a ysignalshifted for examplefrena 153640'te 15,640.5 kilocycles'.`A` Theoscillators aredcsigned's that their actual frequency-difference isequal to the range of audio t'onesyvhiclritis desired to transmit andreproduce at the receiver.' Forfexampl'e, inthe case' of facsimiletransmission; i ar suitable y facsimile scanning arrangement 3`is'provided'whereby the shades ofthe original subject matter'areconvertedvinto-currents?which,` attire receiver, will appear' asvaudidr'equenCytones Afor example" 1000 to 15000. P, S.` Thus thelowest"`cur rent may "represent f whit'e while the `highest currertmayrepresent "black'for` Avice V:versati If ldesir-d, :although it isnotriec'essary,` the converter ymay be adjusted As o'that for therv'angefof, shadestobe.

covered, the uppermost audio frequencyv tone, `or the uppermost portionof the-1000 to 1500 CIP. S; spectrum, is always less thanmthe secondharmonic of the lowermost torre-and therefore vless thanwthelsecondharrrn'n'nic 'of anyy Itoneoi'` frequency inthe said,s1:'e'ctrum.V `Fora 'detailed dey scripti'on of 'such an"arranjgement,reference may beliid' to 'Pat-elfltr No.'2,299,937,"thedisclosure 0fwhich is herein incorpora-ted byvrefe'renc'e.

'The two'radio frequencies from"`oscillators l and2 rare fed to a commonrradiofrequencyltransmittin'g4 amplifier fl whose 'outputiis applied toa radiating antenna 5. Amplifier ishould be' of the distortionless typesuch as a class B ampli-` fier." The two radio frequencies willtherefore be radiated as two discrete frequencies. At the receiverterminal the two radiated frequencies arel picked up by the receivingantenna y'6 to which is connected an ordinary radio receiver of theamplitude-demodulationv type. For illustrative purposes, asuperheterodyne receiver is shown comprising the radio frequencyamplifier section l'1; the first detector or intermediate frequencyconverter section `8; the intermediate frequency amplifier section 9;the detector section I0 such as usedin detecting amplitude modulation;and

suitable audio frequency amplifier section (notshown). Assuming theoriginal signals tobe in the G-1500 C. P. S. band, the receiver is tunedto the mean frequency between the two radiated frequencies of 15,640 and15,640.5 kilocycles. Un-

der those conditions, when the variable oscillator rests at 15,640kilocycles, the audio frequency output 0f the receiver is 1000 C. P. S.When the variable oscillator is shifted to 15,640.5 kilocycles, thereceiver output is 1500 C. P. S. This 1000 to 1500 C. P. S. signal canthen be used to control the reproduction of the original facsimileshades in the Well-knownk manner and as explained in said Patent No.2,299,937.

Preferably, and as described in said patent, the output of the radioreceiver may be passed through a band-pass lter having an eifective bandpass of 1000 to 1500 C. P. S., and it is thenv subjected to a leveluniforming or limiting device to bring all the frequencies to the sameam-` As indicated in Fig. 3, instead of using -a single transmitter toradiate the two radiofrequencies from a single antenna, two entirelyseparate transmitters and antennae may be employed. With such anarrangement, class C amplifiers may be used to amplify the radiofrequency carriers before application to the antenna. Whereas, in thecase of Figs. 1 and 2, it may be necessary to use class B linearampliers which are more critical than class C amplifiers.

It will be understood that each transmitter in the system of Fig. 3, canbe independently keyed for separate telegraph message transmission orthe like when the simultaneous use for facsimile as above described, isnot required.

quency oscillations, generating other primary radio frequencyoscillations, shifting the frequency of said other oscillations withoutaudiofrequency amplitude modulation so that the actual frequencydifference with the rst radio frequency oscillations equals thefrequency of the tone to be transmitted, and radiating both said radiofrequency oscillations from -a common antenna.

4. The method of radio carrier transmission which comprises, generatinga rst primary radio frequency carrier, generating a second primary radiofrenquency carrier, varying the relative frequencies of said carrierswithout audio-frequency amplitude modulation of the carriers so that theactual frequency difference is in a predetermined audio frequencyband,and radiating said carriers from separate antennaefto' a commonreceiving point. 5. A primary radio frequency oscillator of xefrequency, another primary radio frequencyoscillator of shiftablefrequency, means to shift the frequency `of said other oscillatorWithout audio frequency carrier amplitude modulation-and under controlof signals to be transmitted so that the actual frequency differencebetween said oscillators equals the frequency of the signal-to'betransmitted, an amplifier for both of said oscillators, and a commonradiating antenna fed from said amplifier. y

6. 1n a radio transmission system of the charr acter described, a pairof primary radio frequency oscillators, means to vary the frequencies ofsaid oscillators Without audio-frequency amplitude modulation thereofbut under control of4 audio frequency signals so that the frequencydifference between the oscillators equals the frequency of the signalsto be transmitted, a common amplifier for both of said oscillators, anda common radiating antenna fed from said amplifier.

RICHARD A. HILFERTY

